A wide variety of low-pressure discharge lamps are known in the art. These lamps use mercury vapor to generate UV radiation that is converted to visible light by a suitable fluorescent coating of the lamp envelope. In order to achieve best luminous performance of the lamp, the mercury vapor pressure has to be kept at a predetermined value with only little variation. In order to control the mercury vapor pressure, mercury is located in a mercury reservoir/container at a location away from the heated electrodes. Mercury vapor may be provided by liquid mercury or a mercury alloy also called amalgam. As the pressure of mercury vapor of such an amalgam at a given temperature is lower than the mercury vapor pressure of pure liquid mercury, amalgam proves as an ideal mercury source for compact fluorescent lamps (CFL-s), which are exposed to higher operation temperatures due to their smaller dimension. CFL-s typically have a mercury reservoir temperature of above 50° C. The amalgam is optimally positioned near a tip of the exhaust tube.
U.S. Pat. No. 6,597,106 discloses a compact fluorescent lamp with a housing structure including a plastic cap and a plastic socket. The sealed ends of the discharge tube arrangement are received in the cap having contact members, and a protruding fitting member for fitting in the socket. The socket has a hollow member for receiving the fitting member of the cap, and contact elements for receiving the contact members of the cap. The compact fluorescent lamp further comprises a tabulation, which contains amalgam material and communicates with the discharge tube.
Such lamps are widely used in private area and commercial places where energy saving and high luminous efficacy combined with a relatively long lifetime are important. It has however been observed that such lamps are more sensitive to ambient temperature than incandescent lamps. Low-pressure fluorescent lamps and particularly compact fluorescent lamps primarily are to be used in buildings and operated at room temperature in order to provide an optimum of luminous output. Even if used in buildings at regulated room temperature, fluorescent lamps may be exposed to abrupt changes of ambient temperature resulting in heat shocks that have a negative impact on the luminous output. Temperatures substantially below or above the room temperature may result in a substantial drop of luminous intensity of such lamps.
Therefore, there is a need for a fluorescent lamp configuration with a cap and socket arrangement, which exhibits improved luminance, e.g. which is less sensitive to changes of the ambient temperature and therefore no substantial difference in the luminous output of the lamp may be perceived when the lamp is operated under changing ambient temperatures due to positioning of the lamp. More specifically, there is a need to provide a lamp configuration, which does not exhibit a significant decrease in the luminous efficacy when operated in any position, also including a horizontal, base-up or a base-down position. Therefore a lamp configuration is required, which has an improved control of amalgam reservoir temperature for optimum performance of the lamp.